Because in most cases the induction and exhaust are close enough so the dynamic compression at 100% VE can be disregarded. These results are not calculated but empirical. They come from running conditions, and in most cases the running conditions these engines are put under are similar enough where you only need static and dynamic compression.

VERY well tuned engines or poorly tuned engines fall outside of these rules. There are some engines out there with 9:1 compression that require race fuel because their induction system is so well tuned they can see VE over 125%.